Pii: s0003-2670(02)00628-

Analytica Chimica Acta 468 (2002) 119–131
Fábio R.P. Rocha , Boaventura F. Reis , Elias A.G. Zagatto ,
José L.F.C. Lima , Rui A.S. Lapa , João L.M. Santos
a Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, P.O. Box 96, Piracicaba, 13400-970 SP, Brazil
b Faculdade de Farmácia, Universidade do Porto, Rua Anibal Cunha 164, Porto 4050-047, Portugal
Received 2 May 2002; received in revised form 25 June 2002; accepted 4 July 2002
Abstract
Multicommutation refers to flow systems designed with discrete computer-controlled commutators resulting in flow net-
works in which all the steps involved in sample processing can be independently implemented. The flow systems can bere-configured by the control software, presenting thus increased versatility, potential for automation and for minimization ofboth reagent consumption and waste generation. The main objective herein is to review the concept of multicommutation inorder to permit a proper evaluation of the characteristics and potentialities of the related flow systems, to assist methodologicalimplementation and to discuss similarities with other existing strategies. Implementation of tandem streams, controlled dilu-tions, wide-range determinations, sequential determinations, titrations and in-line separation/concentration are emphasized. 2002 Elsevier Science B.V. All rights reserved.Keywords: Multicommutation; Flow analysis; Tandem streams; Automation
1. Introduction
tion by the environment (and vice versa) is avoided.Other features inherent to flow analysis are the low
Since the inception of flow analysis in the earlier
consumption of sample and reagents, the partial and
fifties flow systems have proved to be excellent
reproducible development of the involved steps, that
tools for solution handling and consequently for car-
opens the possibility of reaction kinetics exploitation,
rying out methods related to wet chemical analysis.
the recording of a transient signal, etc.
As a rule, the aqueous sample is introduced into the
In the earlier air-segmented flow analyzers, a sam-
analytical path and processed inside it under repro-
pling arm was used to select either the sample or the
ducible conditions. The manifold can thus be regarded
wash (carrier) solution to be aspirated towards the
as a closed laboratory where the sample contamina-
analytical path This task can be considered as thebeginning of commutation in flow analysis. However,little evolution in the commutation concept was noted
∗ Corresponding author. Tel.: +55-19-429-4650;
during the development of the segmented-flow analyz-
ers. In fact, chemical processing was not altered from
E-mail address: [email protected] (E.A.G. Zagatto).
1 Present address: Instituto de Qu´ımica, Universidade de São
sample to sample and no active devices or feedback
mechanisms were present in the flow system.
0003-2670/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 0 3 - 2 6 7 0 ( 0 2 ) 0 0 6 2 8 - 1
F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131
Commutation became more evident in relation to
unsegmented flow-analyzers conceived in the 1970sSteps others than single sample introduction,such as multiple injections, stream splitting, addition/removal of manifold components and stream redirec-ting were efficiently accomplished by using speciallydesigned commutators. This led to the efficient imple-mentation of merging zones, zone sampling, stopped-flow,
procedures exploiting differential kinetics, simultane-ous determination, etc. A comprehensive review ofcommutation in flow-injection analysis was presentedin 1986
The single commutators, usually sliding bars or
rotary valves, were only able to linked commutationbecause they operated in two resting positions thusoriginating two states. Therefore, in spite of the above-mentioned capabilities, the related systems lacked ver-satility. The drawback was circumvented by designingthe manifold using several commutators with discreteoperation This led to a drastic improvement insystem performance, as 2n states of commutation(n = number of active devices) could be establishedand exploited. The related flow systems, often namedmulticommuted ones, present potentialities to be usedas general-purpose systems, as they rely on the con-
Fig. 1. Representation of some mechanical commutations in flow
cepts of different flow-analyzers (segmented-flow,
analysis. (a–d) Dashed lines represent the flow paths after com-
flow-injection, sequential injection) and are compat-
ible with streams of different characteristics (seg-mented, unsegmented, monosegmented, tandem).
Although, some potentialities of the flow systems
The different configurations in illustrate me-
exploiting multicommutation were recently outlined
chanical commutations in flow analysis. In addition,
the related concepts, adherence with the already
flow rate modifications and stream reversals are also
proposed flow-analyzers and guidelines for system de-
relevant examples usually accomplished without man-
sign were not emphasized. The main objective herein
is therefore to review the concept of multicommuta-
The simplest mechanical commutation involves
tion in order to permit a proper evaluation of the char-
acteristics and potentialities of the related flow sys-
easily accomplished by using three-way valves or
tems, to assist methodological implementation and to
portions of more elaborated valves The strategy
present the similarities with other existing strategies.
has been exploited for intermittent addition of flowingstreams. In this way, a washing stream can be added(after achievement of the analytical signal in
2. The concept of commutation
order to reduce washing time, thus improving samplethroughput , a confluent stream can
Commutation is defined as “(1) a passing from one
be added only when the processed sample is passing
state to another, (2) the act of giving on thing for an-
through a confluence point, allowing reduction of the
other, (3) the act of substituting, (4) in electricity: a
reagent consumption Moreover, the stopped-flow
change of direction of a current by a commutator”
approach can be implemented in a similar manner.F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131
The sample carrier stream of a single-line flow system
expanded in a pronounced manner by taking advan-
is allowed to recycle, and the processing sample is
tage of discrete devices with independent operation.
stopped inside the analytical path. This permits long
This is the essence of the multicommuted systems.
residence times to be efficiently attained in order to
Their inherent potentialities, also in relation to the
improve sensitivity and/or to permit kinetic measure-
exploitation of feedback mechanisms, are discussed
ments On the other hand, this simple mechanical
commutation permits to feed the analytical path withstacks of different zones (The flow-set upis designed in such a way that either stream IN1 or
3. The concept of multicommutation
stream IN2 is directed towards the outlet. The strat-egy is often used in sandwich techniques
A flow system can be regarded as a compart-
sequential injections Expansion of this strategy
ment with a number of inlets (samples, reagents,
permits also the establishment of tandem streams, a
keyboard commands) and several outlets (results,
novel approach for handling flowing solutions. Even
recycled solutions, wastes). When commutation
with a single device, fast successive commutations
is not so characteristic, such as in the classical
(multicommutation) can be performed, originating a
segmented-flow-analyzers, influence of outlet on the
binary string constituted by slugs of the involved solu-
inlet parameters and vice versa, as well as real-time
tions. An analogous strategy permits the achievement
modifications in the sample processing, are not easily
accomplished. With a single commutation, only few
Other kind of mechanical commutation involves
of the potentialities inherent to flow analysis are feasi-
the exchange of manifold components (that
ble These capabilities are expanded in relation to
is a powerful tool for achieving different sample
the most advanced systems, the multicommuted ones.
residence times. In fact, two different mean sample
A multicommuted system can be then considered
residence times can be provided for every assayed
as an analytical network that involves the actuation of
sample depending on whether a short or a long re-
n active devices (or n operations with a single device)
actor is placed in the analytical path. The approach
on a single sample allowing the establishment of up to
is beneficial mainly for implementing simultaneous
2n states. It presents several in- and outlets parame-
determinations involving kinetic discrimination or
ters that are often interdependent. The analytical steps
wide-range determinations Another possibility
required for sample processing can be defined through
is to trap the processing sample inside an incubation
the control software, being eventually (in feedback
coil in order to get increased residence times without
exploiting systems) real-time modified. In short, mul-
ticommutation is inherent to flow systems that may
An usual strategy for mechanical commutation in-
present several states; the sample under processing
volves two inlet and two outlet streams and offers the
is usually submitted to different operations, such as
possibility of selecting different paths between them
splitting, slicing, trapping, mixing in tandem, under
(The configuration can be settled by using
different conditions. Commutation is also responsi-
six-port valves or sliding-bar injectors. As different
ble for additions of components (including sampling
components can be placed in the paths between the
loops) to the analytical path and/or stream re-directing.
commuting sites, the strategy is very attractive for
More elaborated systems usually comprise more
implementing loop-based injection ion-exchange
active devices. Therefore, presence of several valves
involving addition/removal of minicolumns mul-
in the system has been erroneously taken as an indica-
tisite detection leaping filters etc.
tor of a multicommuted system. This is not a sine qua
The above-mentioned potentialities are expanded by
non condition, as there are multicommuted systems
replicating the unity configurations and/or including
with a single valve flow systems with several
feedback mechanisms. When the unity configurations
commutators where the aspect of multicommutation
are replicated in a single commutation unity, only two
is not highlighted t is important to emphasize
states can be established for the system, and linked
that the configuration inherent to sequential injection
commutation is concerned. System versatility can be
analysis is considered here as an expansion of the
F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131
can be introduced into the manifold by rapidly andsequentially switching the commutators, usuallycomputer-controlled valves. This unique stream canbe seen as a set of neighboring solution slugs thatundergo fast mixing while being transported throughthe analytical path.
For constant total volumes of sample and reagent,
mixing is improved by decreasing the aliquot volumesand increasing the number of slugs. Insertion of n pairsof sample/reagent slugs results in 2n − 1 interfaceswhere mixing occurs by axial dispersion. In contrast tomost flow systems, sample/reagent interaction starts inthe sampling step, thus increasing the mean residence
Fig. 2. Establishment of a tandem stream. S1, S2: miscible solu-
time without affecting sampling rate.
tions, V: three-way solenoid valve, R: reactor coil, D: detector.
Tandem stream was initially exploited for the deve-
lopment of an improved single-line system for spec-
commutation concept, although multicommutation is
trophotometric multiparametric analysis of natural
waters into which several plugs of differentreagent solutions were sequentially introduced. Theingenious strategy involved three-way computer-cont-
4. Tandem streams
rolled solenoid valves and permitted the addition of di-fferent
One of the main potentialities of multicommutation
is the establishment of a tandem stream (
A similar strategy named tandem injection was
A number of aliquots of different miscible solutions
adopted for sample dilution prior to sequential ICP–
Fig. 3. Implementation of tandem streams: (a) carrier flowing towards the analytical path; (b) insertion of a sample aliquot; (c) insertionof a reagent aliquot in tandem with the sample; (d) insertion of a sample aliquot in tandem with the reagent. Vi: commutators; C: carrier;R: reagent; D: detector. For didactic purposes, dispersion at the liquid interfaces (as shown in sample/reagent mixing wasomitted.F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131
OES or ICP–MS Several sample plugs were
5. Flow setups
introduced into a diluent solution yielding a tandemstream that was directed towards the inlet of the
Multicommutation is usually accomplished by tak-
nebuliser. Under good mixing conditions, an almost
ing advantage of valves, timing devices and other
steady situation was attained. Undulations on the
artifacts for improving system performance. These
recorded tracing were reported, and procedures for
devices can be operated in a passive or active manner,
minimizing them were discussed. Another way to
and external timing is often exploited for versatility
implement a tandem stream is to converge different
flowing solutions towards a rapidly and sequentiallyswitching three-way valve. This is particularly attrac-
tive when the tandem stream feeds the sampling loopof a flow-injection system the holding coil
In a multicommuted flow-analyzer comprising only
passive devices, sample processing is defined pre-
Drawbacks caused by differences in refractive
viously to its introduction into the analytical path.
index in spectrophotometric measurements can be
In spite of this, several time-dependent analytical
readily minimized in flow-systems with tandem
procedures can be efficiently carried out, as the sys-
streams, as verified in the determination of pindolol
tem presents inherent timing. The discrete devices
in pharmaceutical preparations and ethanol in
are thus used mainly to establish tandem streams,
alcoholic beverages Multicommuted flow sys-
merging zones, addition/removal of components, etc.
tems with tandem streams were also proposed for
in order to improve figures of merit such as reagent
improving sample/reagent mixing in the spectropho-
consumption and sample throughput. Versatility is
tometric determination of ascorbic acid and
improved because the devices are usually indepen-
clomipramine in pharmaceutical preparations,
dently operated. Implementation of zone sampling is
and glycerol in alcoholic fermentation juices
a good example to illustrate this feature: with linked
Other profitable examples are presented in the next
mechanical commutation, only one sample aliquot
per injection can be re-sampled whereas a number of
The previously described approach has been dif-
aliquots per injection can be obtained by exploiting
ferently named. Israel et al. termed their proposal
as tandem injection The term binary sampling
Tandem streams are also compatible with flow net-
was adopted in the first article of the series about
works based exclusively on passive devices. In this
multicommutation expression multi-insertion
way, the intermittent addition of reagents was ex-
principle was adopted when a procedure for the de-
ploited for the determination of nickel with dimethyl-
termination of nitrate and nitrite in water, fertilizer
glyoxime. Different reagent plugs were inserted in
and food samples was proposed However, the
tandem with several sample aliquots, allowing the
authors termed a similar strategy as tandem flow when
establishment of a stream comprising the solutions
proposing a procedure involving gas diffusion for
required for iron masking, nickel oxidation and devel-
determination of chlorine A similar approach
opment of the color-forming reaction A similar
named pulsed flow the insertion of
strategy involving immobilization of the oxidizing
small aliquots of different solutions at a frequency
typically 1.0–1.5 Hz. Solutions are pressurized against
The potentialities of the analyzers are expanded by
nozzles in order to attain instantaneous turbulence
taking advantage of external timing. In this way, the
that contributes to improve mixing and reduce axial
different steps of sample processing (addition of spe-
dispersion. The authors coined the term time-division
cific reagents for sequential determinations, multiple
multiplex technique for an approach that consisted
stop periods, cascade dilutions, etc.) can be indepen-
in using computer-controlled solenoid valves for
dently implemented. Another possibility is to provide
creating concentration profiles exploited for poten-
via keyboard the information needed for processing
tiometric titration of calcium and spectrophotometric
different samples in a diverse and specific way as in
F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131
Incorporation of feedback mechanisms in the
Dilution is inherent to flow systems being dictated
control software expands the performance of multi-
by parameters such as sample volume and analytical
commuted flow-analyzers. With active devices strate-
path length. Moreover, the dilution degree can be
gically placed in the manifold, sample processing
increased by exploiting zone sampling split
can be modified according to the preliminary mea-
zones for example. In this way, multicommuted
surements. In this way, parameters such as sample
systems have been proposed to implement controlled
residence time or sample dispersion can be easily
dilutions in order to expand the concentration range of
modified, as demonstrated in the wide range determi-
procedures, thus avoiding outlier samples. In this con-
text, a flow system was proposed for controlled dilu-
monitored signal is acceptable for providing the ana-
tions of plant digests aiming the direct determination
lytical result, another measurement can be performed
of calcium by flame atomic absorption spectrometry
and potassium by flame atomic emission spectrometry
A similar strategy can be exploited in designing in-
The diluent aliquots were intercalated in tan-
telligent systems, such as the flow-injection system
dem with the sample plugs, and each sample/diluent
proposed for the analysis of saline waters by ICP–OES
volumetric ratio corresponded to a different dilution
Matrix interferences caused by differences in
degree. The approach allowed up to 40-fold dilution
salinity between sample and reference solutions were
without affecting the analytical precision.
circumvented by considering a prior measurement that
An alternative approach was proposed for the fluo-
allowed the proper selection of the amount of sodium
rimetric determination of folic acid in pharmaceutical
chloride to be added to the reference solutions.
Self-optimized flow systems are also feasible, as
employed to change the sample volume in order to
the operational conditions can be systematically varied
provide variable degrees of sample dilution, thus ex-
according to a previously defined algorithm in order
tending the linear response range. Samples were firstly
to attain the best analytical response Moreover,
processed in a condition of medium dispersion and
discrete devices can be exploited for implementing
the analytical signal was evaluated by the control soft-
variations in flow rates, stop intervals, flow reversal,
ware that decided if the analyte could be quantified
etc. They may also be useful for specific applications
or if the sample should be re-processed with higher
involving localization/processing of a flowing sample
or lower dispersion. In this way, linear response was
obtained within 0.100 and 40.0 mg l−1, yielding a pro-
discussion of this possibility is outside the scope of
cedure able to monitor tablet dissolutions.
An automated multicommuted flow system was
proposed for expanding the linear range in spectropho-tometric procedures, aiming the determination of cal-
6. Applications
cium in different samples ive sample-processingconditions were previously defined, corresponding to
As science is recurrent, it is very difficult to pre-
dispersion coefficients within 2.15 and 754, achieved
cisely define when multicommutation was conceived.
by changing the sample volume (10–500 ␮l), the an-
In fact, several contributions involving different in-
alytical path length (225–425 cm) and by exploiting
dividually operated discrete devices a number
zone sampling at different portions of the dispersed
of linked three-way valves several commutators
zone. The criterion for accepting a given measurement
establishment of tandem streams etc.
or to call for sample reanalysis was similar to that
were proposed before the term multicommutation was
previously described. Quantification was achieved in
coined. However, the applications highlighted below
up to three trials, and the R.S.D. were estimated as
are restrict to those where reference to multicommu-
<0.8% regardless of the sample processing condi-
tation is explicit and/or its main characteristics are
tions. Linear response within 0.250 and 1000 mg l−1
allowed the direct determination of calcium in waters,
F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131
plant digests, antacids, fertilizers and calcareous
was proportional to zinc concentration due to the
rocks. Multicommutation was also exploited to ex-
limited extension of the copper reaction whereas the
tend the linear response range for the turbidimetric
following analytical signal reflected the concentration
determination of sulfate in plant digests the
of both species. Intermittent reagent addition allowed
fluorimetric determination of isoniazide in pharma-
also the design of a flow system for the determina-
ceutical preparations Also, a similar approach
tion of iron and aluminum in plant digests, using
allowed implementing programmable isotope dilu-
tions for ICP–MS Spikes were generated from
The strategy was expanded when a multicommuted
a single enriched 112Cd solution. About 30 sam-
flow system was proposed for spectrophotometric
ples could be run per hour consuming 33–168 pg of
determination of total nitrogen and phosphorus in
biological materials involving intermittent additions
A flow system designed in the closed-loop configu-
of several reagents and requiring increased resi-
ration was proposed for implementing successive dilu-
dence times The manifold was designed with
tions required for expanding the concentration range in
a single channel into which sodium salicylate and
the chloride determination in parenteral solutions
sodium hypochlorite or ammonium heptamolybdate
The sample zone was continuously recycled through
and ascorbic acid were added by proper switching
the flow cell, allowing several analytical signals to be
of strategically positioned solenoid valves. Moreover,
obtained for the same sample injection. After obtain-
the alkaline and acidic conditions required for the
ing the first analytical signal, the front and tail portions
different determinations were efficiently established.
of the sample zone were removed and replaced by the
Although an ammonium salt was used for the deter-
reagent solution, by the action of a three-way solenoid
mination of phosphorus, the analytical signal corre-
valve. Thus, the sample dispersion and the reagent
sponding to nitrogen was not hindered by carryover
availability were both increased. The process was re-
effects, emphasizing the capability of the strategy in
peated several times, resulting in signals correspond-
dealing with addition/removal of reagents. The sam-
ing to different dispersion degrees, allowing achieving
ple residence time was increased by decreasing the
linear response up to 10 g l−1, with R.S.D. <1.8%.
rotation speed of the computer-controlled pump afterthe sampling step. A multicommuted flow system
was also employed for the sequential potentiometricdetermination of free and total cyanide exploiting gas
Discrete commutators with independent computer-
control allow the intermittent addition of different
The potentialities of multicommutation for sequen-
reagents, thus expanding the potentiality for sequen-
tial determinations are expanded by association with
tial determinations without changing the manifold
multichannel detectors. Spectrophotometric multi-
structure. The strategy can be illustrated by the mul-
determinations using a low selectivity chromogenic
tiparametric analyses of alloys Aliquots of the
reagent (4,2-pyridylazoresorcinol) were implemented
reagents required for iron and chromium determina-
by exploiting measurements at different pH values,
tion were introduced in tandem with sample slugs
addition of masking agents and kinetic/spectral dis-
permitting the sequential determination.
crimination for the sequential determination of iron,
Multicommutation was also exploited for simul-
copper, zinc and nickel ver, a flow-system
taneous determination of copper and zinc in plant
with intermittent addition of different selective chro-
digests using Zincon as chromogenic reagent
mogenic reagents was proposed for the determination
The analytes were in-line complexed with cyanide
of iron, copper, zinc, calcium and magnesium in
and the different rates of reaction with formaldehyde
multivitaminic preparations Sample/reagent in-
allowed implementing the kinetic discrimination. A
teractions were improved due to the establishment of
three-way solenoid valve was used for managing the
sample splitting, yielding two sample aliquots to be
Multicommutation permits different tasks to be
processed under different conditions. The analytical
simultaneously accomplished for improving system
signal corresponding to the shorter residence time
performance and analytical characteristics. In this
F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131
sense, a flow system was proposed for the inorganic
Multicommutation in connection with the stopped-
speciation of nitrogen in waters For ammonium
flow approach was exploited for the spectrophotomet-
determination, the processing sample was trapped in-
ric determination of creatinine in urine using picric
side a coiled reactor in order to improve the reaction
acid as reagent Design of the flow manifold al-
development. Simultaneously, another sample aliquot
lowed improving analyte conversion rate and in-line
was processed for nitrate and nitrite. In this way,
sample dilution was achieved through zone sampling.
60 determinations could be performed per hour with
In this way, matrix effects were minimized and the
a 120 s residence time for ammonium, enough for
dynamic response range matched the expected crea-
achieving a 95% conversion rate. Other application
tinine concentrations in the samples. As a high blank
in this context refers to the sequential determination
value was concerned, an ingenious strategy permitted
of anions in natural waters The sample acted
the correction of the analytical signal. The sample
as carrier solution into which several plugs of the
aliquot was split and one portion was retained inside a
required chromogenic reagents were introduced. The
warm reactor (37 ◦C) in order to enhance the reaction
system allowed also in-line concentration into an
development, whereas the other was straightforward
anion-exchange resin minicolumn simultaneously to
directed towards detection for evaluation of the blank
the direct determination of the analytes in order to per-
mit species at lower concentration to be determined.
The beneficial effects of implementing zone
Multicommutation allows also implementing pro-
trapping with multicommutation were illustrated in
cedures for accuracy assessment. The analyte is de-
relation to an improved procedure for the spectropho-
termined by two different methods with the same
tometric determination of phoshate in natural waters
flow network and the mean of the obtained results
Further, simultaneous trapping of multiple sam-
is inherently more reliable. In this way, a multi-
ple zones were utilized to permit an increase in the
commuted flow manifold was proposed for chloride
sample residence time without impairing the sampling
determination in natural waters with different sample
rate in a flow system designed to spectrophotomet-
matrixes based on the spectrophotometric proce-
ric determination of boron in plant digests A
dure using mercury(II) thiocyanate and the turbidi-
four-way solenoid valve directed the sample zones to
metric method using silver nitrate Moreover,
three similar reaction coils allowing a mean residence
in-line addition/recovery tests were implemented
time of 200 s with a sampling rate of 65 determina-
for every assayed sample in order to detect matrix
tions per hour. The favorable characteristics of the
approach were also highlighted in the determina-tion of ammonium and phosphate in natural waters
using an electronically operated sliding bar commu-tator coupled to three-way solenoid valves The
For improving sensitivity in analytical procedures
sample zones were simultaneously processed and a
based on relatively slow reactions, the system is de-
sampling rate of 56 determinations per hour was at-
signed to provide an increased sample processing time
tained even for a 160 s residence time. Recently, the
without impairing other features such as sampling rate
strategy was applied to the spectrophotometric deter-
and sample dispersion. In this regard, the stopped-
mination of amiloride hydrochloride in pharmaceutic
flow approach typically involves monitoring
preparations. With two channels able to accommo-
of the processed sample after stopping it inside the
date stopped sample zones, the sampling rate and the
flow-through detector presents unique advantages.
mean sample residence times were 30 h−1 and 60 s,
Also, other flow setups allowing sample trapping in-
respectively. It is interesting to comment that these
side specific portions of the manifold have been pro-
figures of merit are modifiable at will depending on
posed Alternatively, increasing the sample
processing time can be efficiently accomplished by
Multicommutation has also been exploited to fa-
exploiting segmentation The potentialities of
cilitate the introduction of sample, reagent and air
these approaches are expanded with multicommuta-
bubbles into a monosegmented flow system The
approach was demonstrated in the spectrophotometric
F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131
determination of manganese in soybean based on pe-
used as carrier stream and the reference signal was
riodate oxidation of Mn(II) to permanganate in acidic
estimated during its passage through the electrode.
medium. Improved sensitivity was attained by setting
The volume of the sample zone was fixed during the
a 5 min sample residence time and immersing the
entire titration procedure, thus avoiding matrix vari-
main reactor inside a 47 ◦C water bath. The reaction
ations that could affect the electrode response; the
was completed before detection and a sampling rate
titrant volume was continuously varied therefore a
of 50 determinations per hour was achieved because
diluent solution was added to keep the total volume
four sample zones were simultaneously processed.
of the processed sample. With monosegmentation, the
A similar strategy was adopted in a flow system for
mixing conditions were improved without excessive
spectrophotometric determination of l(+)-lactate in
sample dispersion. The search process was analogous
silage material using lactate oxidase and a crude
to the molar ratio method for determining complex
extract containing peroxidase The system pro-
stoichiometry. At every step, the sample zone was
vided a very long sample residence time (17 min) and
monitored and the signal evaluated in order to decide
permitted 16 samples to be processed per hour.
if the titrant volumetric fraction should be increasedor decreased. For achieving the end-point, the signal
should be within the pre-selected interval defined asthe mean value ± three-fold the standard deviation of
Titrations are classical procedures often required
the measurements performed on the buffered carrier
in analytical laboratories in view of the achievement
stream. The procedure yielded precise (R.S.D. = 1%)
of intrinsically reliable results. As they are generally
and reliable results even for titrations of dilute weak
tedious and time-consuming, several mechanized pro-
acids. A similar algorithm was further applied for the
cedures have been proposed, including those carried
determination of ascorbic acid in juices and soft drinks
out in a flow basis However, most of them can-
involving titration with 2,6-diclorophenol-indophenol
not be rigorously considered as titrations, because a
tation and tandem streams were also exploited to
True titration procedures, without requiring cal-
develop a procedure for acid–base titrations based on
ibration, have been developed by exploiting multi-
commuted flow setups. In this context, the binary
An ingenious approach involving one three-way
search process was proposed for end-point detec-
valve that managed sample and the titrand inlet as a
tion in spectrophotometric titrations It relies on
tandem stream was proposed and applied to acid–base
the evaluation of the least volumetric fraction of the
titrant causing a measurable change in the color ofan indicator. Signals related to sample zones com-
6.5. In-line concentration/separation
prising only the sample or the titrant were consideredas reference. For end-point detection, the sample and
Procedures for in-line concentration are usually
titrant volumetric fractions were varied by maintain-
time-consuming, therefore impairing the sampling
ing the total volume of the sample zone, similarly to
rate. The drawback can be circumvented by using a
the continuous variation method. Feedback mecha-
multicommuted flow manifold, as illustrated in the de-
nisms assisted the decision about whether sample or
termination of cadmium, nickel and lead in foodstuffs
titrant volumetric fraction should be increased in or-
and plant materials by ICP–OES sorption
der to converge to the end-point. The procedure was
and elution were simultaneously carried out by using
illustrated by the acid–base titration in the presence
three ion-exchange columns managed by a four-way
of phenolphthalein. End-point was reached in up to
valve, allowing a sampling rate of 90 determinations/h
3 min and less than 2 ml of titrant was consumed.
to be attained even for a 120 s loading time.
The binary search process was further applied to
Multicommuted flow-systems were also associated
the potentiometric acid–base titration, employing a
to an electrothermal atomic absorption spectrome-
tubular polymeric membrane hydrogen ion-selective
ter with a tungsten-coil atomizer, aiming to perform
electrode A diluted buffer solution (pH 7.0) was
in-line concentration and separation Analyte
F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131
retention/elution, system washing and column con-
sampling strategies such as tandem streams, sandwich
ditioning were independently accomplished by using
sampling, monosegmented flow and sequential injec-
discrete commutators. The potentialities were illus-
tion analysis were implemented without changing the
trated by in-line concentration of lead in a Chelex-100
resin minicolumn matrix separation by sorp-
Minimization of the reagent consumption is one
tion of metal complexes in fullerene (C60) aiming
of the favorable characteristics of the multicommuted
the determination of cadmium, lead and nickel in
flow systems, especially those exploiting tandem
water samples A similar flow setup was pro-
streams. A considerable reduction in the reagent con-
posed for in-line separation-concentration of copper
sumption and waste generation is the most direct
in the analyses of unpolluted seawater by graphite
alternative towards the development of green analyt-
furnace atomic absorption spectrometry with a W-Rh
ical procedures This can be exemplified by the
enzymatic determination of glucose in soft drinks and
A flow system designed with discrete commutators
sugarcane juices where the required amounts of per-
(three-way solenoid valves) was employed for in-line
oxidase e glucose oxidase were 85% lower in relation
matrix separation and concentration of copper, cad-
to the analogous batch procedure The reagent
mium, lead, bismuth and selenium from seawater aim-
consumption and consequent generation of effluent
ing their determination by electrothermal vaporization
volumes associated with different flow-based proce-
ICP–MS In-line resin conditioning, concentra-
dures were critically compared in relation to the de-
tion, column washing and elution were independently
termination of cabaryl with p-aminophenol The
performed, and a sampling rate of 22 determinations/h
multicommuted system required a reagent amount
similar to that of a sequential-injection system, and ca.
A multicommuted flow system was employed for
27-fold lower than that of a typical flow-injection sys-
overcoming the interference of lead and copper on the
tem. However, other analytical figures of merit such
spectrophotometric determination of cadmium
as sampling rate and detection limit were enhanced
Interfering species were separated by electrolytic de-
in comparison with the sequential injection system.
position and cadmium was in-line concentrated as
In addition, improved sensitivity in the spectrophoto-
chloro-complexes in an anion-exchange resin mini-
metric determination of iron in waters in a sequential
column. In this way, up to 50 mg l−1 Pb2+ and
injection system was achieved by exploiting tandem
250 mg l−1 Cu2+ did not interfere in the determina-
streams The potential of the flow-based proce-
tion performed at a sampling rate of 20 h−1 with a
dures, including the multicommuted ones, to follow
detection limit estimated as 0.23 ␮g l−1.
the general tendency towards a greener analyticalchemistry was recently emphasized
System optimization is usually time-consuming,
and can be automatically carried out by exploiting the
In contrast to conventional flow-systems, multi-
independent control of the discrete commutating and
commuted systems exploiting tandem streams permit
propelling devices, as demonstrated for chloride de-
the use of a single channel for handling the dif-
ferent involved solutions This feature allowed
flow-rates, sample/reagent volumes and mean avail-
exploitation of gravity to propel the solutions
able time for reaction development were real-time
aiming the achievement of a pulseless flow and a re-
modified according to a simplex algorithm. The opti-
duced maintenance program. The performance of the
mization process was efficient even for sample batches
system was demonstrated in the spectrophotometric
with pronounced variability in analyte concentrations.
determination of chloride in natural waters with a
The versatility of multicommutation was exploited
sampling rate of 160 determinations/h and reduced
to develop a flow system able to detect and circum-
reagent consumption. Gravity was also the driving
vent potential sources of inaccuracy Feedback
force in the turbidimetric determination of sulphate
mechanisms were included in the control software
in plant and animal tissues By exploiting a
aiming the real time characterization of the sources
computer-controlled six-way solenoid valve, several
of inaccuracy and adoption of corrective actions for
F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131
every assayed sample. Specific situations where the
of the already existing modalities and not a novel one.
analytical results were more susceptible to inaccuracy
The growth of proposals in conexion with multicom-
such as partial overlap of sample and reagent zones,
mutation is then clearly to be increased.
improper matrix matching, incomplete masking as
In view of the presence of discrete active devices
well as lessening of the efficiency of solid-phase
in the analytical path, systems exploiting feedback
mechanisms will probably be more and more de-
Multicommuted flow systems with detection by
signed especially with regard to intelligent systems,
chemiluminescence were proposed for determination
polyvalent systems, total analytical systems, novel
of phenols in natural waters lactic acid in
procedures for in-line solvent extraction and sample
yoghurts The former procedure was based on
preparation, implementation of several methods per
the oxidation of phenols by permanganate in acidic
analyte aiming accuracy assessment, detecting and cir-
medium after in-line concentration on a XAD-4 resin
cumventing sources of inaccuracy, etc. The tendency
minicolumn. A detection limit of 5 ng l−1 was at-
towards miniaturization will certainly be increased.
tained with a sampling rate of 12 determinations/h.
This aspect, together with the more rational utiliza-
The procedure for lactic acid was based on reaction
tion of reagents by multicommuted systems, will lead
with lactate oxidase yielding hydrogen peroxide that
to a reduction in sample and reagent consumption. As
reacted with luminol in alkaline medium producing
a consequence, a decreased waste generation is fore-
the chemiluminescence. The sampling rate was esti-
seen, matching the present tendency towards Green
mated as 55 determinations/h for samples containing
within 10–125 mg l−1 l(+)-lactate. All required stepswere efficiently implemented with discrete commu-tators and the procedures presented a considerable
Acknowledgements
reduction in the reagent consumption.
Multicommuted flow systems were designed for
Partial support from FAPESP, CNPq and CAPES/ICCTI
the independent management of the solutions aiming
in-line eletrodissolution of alloys. In this context, thedirect determination of aluminium, copper and zincin the solid sample by FAAS was implemented
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